Pollution by various contaminants of different parts of the environment and the existence in low volumes of valuable metals have caused a variety of chemical removal techniques to be developed. These techniques differ based upon the chemical specie to be removed and the environment from which it is to be removed. For example, the techniques used for the removal of metallic elements from waste water are different from the treatment of inorganic, contaminated soils primarily because of the presence of insoluble, abrasive particles to which the inorganic materials are absorbed or adsorbed.
In the treatment of waste water, two techniques which are conventionally used are metal plating and insoluble precipitate formation. In the metal plating method, an anode and a cathode are placed in the waste water, connected to a DC source and an electric field is created between the electrodes. Certain ions already existing in the water are attracted toward one electrode, and deposit onto that electrode after migrating through the highly viscous waste water. In the precipitate formation method, a cathode and an anode are placed in the waste water and an acid insoluble precipitate is formed at one of the two electrodes. The precipitate is collected and removed from the system by filtration. Patents describing these techniques include U.S. Pat. No. 4,226,685 to Portal et al. and U.S. Pat. No. 3,703,453 to Gordy et al.
Another technique for treating waste water and organic sludge is based upon fluidized bed technology. Anodes and cathodes are placed in waste water, the cathode is in electrical contact with metallized particles in the waste water. An ion permeable membrane separates the anodes from the cathodes. The metallized particles increase the effective surface area of the cathode, and the metal is plated from ions already existing in the solution onto the metallized particles. The metal particles are subsequently removed from the waste water by filtration. Patents describing this technology include U.S. Pat. No. 3,970,531 to Recht and U.S. Pat. No. 3,899,405 to Iverson et al.
A fourth method of removing heavy metals from waste water involves the use of sacrificial electrodes in which insoluble heavy metal precipitates are formed during the dissolution of one electrode and are filtered from the water based system. Patents describing this technology include U.S. Pat. No. 4,318,789 to Marcantonio, U.S. Pat. No. 4,014,766 to Watanabe et al. and U.S. Pat. No. 3,748,240 to Johnson et al.
Losack, in U.S. Pat. No. 5,302,287, teaches to use detergents and nutrients for microorganism growth, and to separate the contaminated soil and water. U.S. Pat. No. 5,405,509 to Lomasney et al. describes an in-situ technique in which metal ions migrate through soil in response to an electric field between a cathode and an anode buried in the soil. These metal ions are entrapped in an ion trapping material or concentrated in the soil near one electrode. The movement of the ions in the soil is substantially slower than the movement of ions in waste water. Furthermore, many methods for in-situ electric field migration of metal ions only concentrate the metal ions in the soil near one of the electrodes, requiring subsequent removal and treatment of the soil.
None of the above described techniques provide for rapidly and efficiently removing heavy metals or other such chemical species from soils which contain matter such as rocks and sand. The waste water treatment techniques, which are relatively rapid, cannot easily be converted to the treatment of soils containing rocks, sand and clay, since the flow of metal ions is substantially slower in the two environments, and since the highly abrasive inorganic matter would cause problems when used with a waste water treatment system. Additionally, waste water ion removal involves removal of existing ions, not the ionization and then the subsequent removal of compounds and other metals in ionic form. Furthermore, many waste water treatment systems use filtration of precipitates or metallized particles onto which the pollutants are plated. The filtration of the precipitate or the metallized particles becomes extremely difficult when rocks and sand exist in the mixture to be remediated, since a filter cannot discriminate between similarly sized particles which should stay in the soil (such as rocks) and particles which should be removed (such as the precipitates). Furthermore, the highly abrasive sand and other inorganic particles tend to abrade and wear metal pieces off the metallized particles and allow the metal pieces to remain in the system, defeating the removal of metallic pollutants from the system.
Therefore, the need exists for a technique for removing metal pollutants from natural materials, such as soil, which does not involve filtration, and which provides rapid, efficient removal of the metal pollutants.